Gravity Survey Included In Christchurch Subsurface Study

GNS
Science will spend the next two weeks undertaking a gravity
survey of Christchurch and the Canterbury Plains to get a
clearer picture of the subsurface structure of the
region.

The project is part of a multi-pronged effort to
learn more about the geological structure under Canterbury
to help with earthquake recovery and rebuilding decisions
in the wake of the devastating magnitude 6.3 earthquake on
22 February 2011.

Information collected during the gravity
survey will be combined with seismic reflection data and
aeromagnetic data to give scientists a clearer view of the
subsurface and help locate any faults.

Several hundred
gravity measurements already exist in Canterbury, some of
them dating back several decades.

However, the spacing
between the measurement points is uneven so the knowledge
of the subsurface geology in some areas is good and quite
poor in other areas.

Project leader, Vaughan Stagpoole of
GNS Science, said the main aim of this initiative was to
achieve more uniform coverage of gravity measurements and
therefore more uniform information about subsurface
geology.

“We hope to collect gravity data at up to 200
locations in Canterbury during the next two weeks,” Dr
Stagpoole said.

The scientists will be using a highly
sensitive gravity meter, which is about the size of a brief
case. They place it on the ground at each location for a
few minutes and record the measurement visually from the
instrument.

“Typically we place the instrument beside a
road or a farm track and it takes just a few minutes at
each location to record the gravity,” Dr Stagpoole
said.

Small changes in gravity measurements over a region
help scientists determine geological structures in the
subsurface. The instrument is sensitive to subtle changes
in gravity that can be associated with the density of
different rock types.

The measurement locations will cover
the region where earthquake activity has occurred since
September 2010, including some locations in the Port
Hills.

The gravity data will augment information from a
seismic survey of Christchurch currently being undertaken
by the University of Canterbury and its Canadian
associates. The seismic data will enable scientists to map
faults that may be hidden under hundreds of meters of
gravels and sediments.

A third leg of the research will
involve an aerial survey of Christchurch to measure
magnetic variations in the subsurface.

This is likely to
involve a light aircraft flying on a grid pattern and
carrying sophisticated magnetic measuring equipment.

The
fourth and final leg of the project is detailed analysis of
all the aftershocks. GNS Science has been undertaking this
work since September 2010. The analysis helps scientists
understand the size and orientation of the faults under
Canterbury. The work is taking place under contract to the
Canterbury Earthquake Recovery Authority and the Ministry
of Civil Defence and Emergency Management. Both
organisations will use the data, along with other
information, to help with planning and rebuilding
decisions.

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